Massive MIMO multi-user beamforming provides the potential to significantly increase the spectral efficiency and throughput by many folds through spatial multiplexing, offering linear capacity growth without the need of increasing the spectral bandwidth. However, when the number of Radio Frequency (RF) chains and antennas becomes large, the overhead in channel estimation to obtain the CSI is significant. Note that it is understood that an antenna or an array of antenna elements is associated with an RF chain, transmitting or receiving, thus, hereafter when the number of antennas is used, it should be understood to indicate the number of antennas and the associated RF chains where each antenna is assumed to be associated with an RF chain. For a Base Station (BS) with a large number of antennas, e.g., M=512 antennas, to simultaneously serve multiple receivers on the same time-frequency resource in the DL, e.g., K=32 User Equipments (UEs) and/or Small Cells (SCs) which depend on a BS to provide wireless backhaul, the BS transmitters must know the CSI of the M×K channel matrix, where M>>K. To be precise, it is the CSI between M BS antennas and the total number of antennas on the K UEs and/or SCs. To simplify discussion, without loss of generality, the total number of UE and/or SC antennas is assumed to be K.
In massive MIMO systems, it is not efficient to obtain the DL CSI directly by sending reference pilots in the DL and feeding them back because of the two following reasons. The first reason is that the large number of antennas on the BS would cause large system overhead for reference signals in the DL. In addition, a large number of bits is needed to quantize the CSI accurately, which would cause infeasible overload of the feedback channel in the UL. Fortunately, the reciprocal property of an over-the-air wireless channel, such as in a Time-Division Duplex (TDD) system or in a Frequency-Division Duplex (FDD) system using switching to create channel reciprocity as described in our patent application PCT/US2014/071752, can be employed to reduce the channel estimation overhead. In this method, each UE and/or SC sends the Sounding Reference Signal (SRS) or pilot signal with a unique sequence in the UL specified resource then the BS estimates the CSI between each pair of transmitting and receiving antennas at the baseband. In FIG. 1, the components of the UL CSI and the DL CSI of a Multi-User MIMO (MU-MIMO) communication system are presented, where the BS 1 is consisted with a baseband processor 2, M RF transmitters 3, M RF receivers 4, and M antennas 5, while each UE 6 is consisted with a baseband processor 2, an RF transmitter 3, an RF receiver 4, and an antenna 5. The BS with M antennas serves K UEs on the same-time resource through the over-the-air channel 7. The responses of the mth BS RF transmitter and the mth BS RF receiver are denoted by tmBS and rmBS respectively, where m=1, . . . , M. The responses of the RF transmitter and the RF receiver on the kth UE are denoted by tkUE and rkUE respectively, k=1, . . . , K. The over-the-air channel between the mth BS antenna and the kth UE is denoted as hm,kair. Hence, as shown in FIG. 1, the CSI measured by the BS actually consists of responses of three components. i.e., the UE RF transmitter, the radio over-the-air channel, and the BS RF receiver, e.g., the measured CSI between the kth UE and the mth BS antenna is written as hm,kUL=rmBShm,kairtkUE, where the UE is assumed to have a single antenna. However, although the radio over-the-air channel is reciprocal between the UL and the DL, the other two components are not reciprocal, which causes the DL CSI to be different from the measured UL CSI, i.e., hm,kDL=tmBShm,kairrkUE≠t hm,kUL. For this reason, before the measured UL CSI is used to calculate the DL beamforming matrix or precoding matrix, some modifications or calibrations have to be done to obtain the estimated DL CSI.
The prior art to solve this problem can be classified into two types as listed below. The first one needs the UE to feed back some related information [1], e.g., the responses tkUE. With the feedback information and the measured parameters of the transmitters and receivers on the BS, the BS can complete the calibration. With this method, the BS could estimate the actual DL CSI up to the accuracy of the measured and feedback information. The second type only needs the BS to measure parameters of the transmitters and receivers on the BS to obtain a scaled DL CSI as in reference [2], i.e., hm,kDL,est=βkhm,kDL, where βk is a complex-valued scaling factor. The first type does not only increase the complexity and cost of a UE, but also causes unnecessary feedback overhead to the networks. The second type needs the BS to measure the responses between a reference antenna and all other antennas in both directions, either over the air or using circuits, which indicates that it can only be completed offline or during idle time considering the large number of antennas. However, as the temperatures of the transmitters or receivers change and components age, the responses tmBS and rmBS would change. Hence, the prior methods for calibrating the UL CSI and DL CSI are either too complex or not accurate enough. For these reasons, this invention provides a novel method and apparatus designed to overcome these shortcomings.